Conventionally, it is well-known that when, for example, a petroleum residue is used as low-grade fuel, vanadium in the petroleum residue fuel generates a low melting compound, and the low melting compound exhibits a binding effect to cause an ash deposition phenomenon to a heat-transfer pipe in a boiler.
An additive such as MgO is added to the petroleum residue. On the other hand, a measure against ash deposition has been taken with respect to a biomass fuel by co-combustion with coal (Patent Documents 1 and 2).    Patent Document 1: Japanese Patent Application Laid-open No. 2002-243108    Patent Document 2: Japanese Patent Application Laid-open No. 2007-107757